1. Field of the Invention
The present invention relates to a distributed amplifier topology and more particularly a distributed amplifier topology with distributed negative feedback which enables the gain of the distributed amplifier to be varied without affecting the bandwidth of the amplifier or significantly affecting the return loss, IP3 or noise figure performance.
2. Description of the Prior Art
Distributed amplifiers are generally known in the art. Examples of such distributed amplifiers are provided in detail in U.S. Pat. Nos.: 4,918,401; 4,947,136; 5,274,339; 5,386,130; 5,412,347; and U.S. Pat. No. 5,414,387. Such distributed amplifiers are also fully discussed in the literature; "MESFET DISTRIBUTED AMPLIFIER DESIGN GUIDELINES"; by Beyer, et al., IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-32, No. 3, March 1984, pp. 268-275; "ON GAIN-BANDWIDTH PRODUCT FOR DISTRIBUTED AMPLIFIERS"; by Becker, et al., IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-34, No. 6, June 1986, pp. 736-738, hereby incorporated by reference.
Such distributed amplifiers are known to be used at microwave frequencies because of the increased gain-bandwidth product at such frequencies relative to conventional amplifiers. A known distributed amplifier is illustrated in FIG. 1. As shown therein, the distributed amplifier, generally identified with the reference numeral 20, includes a plurality of cascaded amplifier stages 22-26, for example, n. Each amplifier stage 22-26 includes a transistor which may be a FET, MESFET or HEMT (herein after referred to as FETS for simplicity) connected in a common source configuration. All of the gate terminals of the FETS are connected together forming a gate line 28. Similarly, all of the drain terminals of the FETS are connected together forming a drain line 30. The input and output capacitance of each of the FETS is combined with lumped inductors formed, for example from microstrip lines, to form distributed impedances, 32-46, thereby forming artificial transmission lines. The use of microstrip lines for the lumped inductors makes the topology amenable to being fabricated as a microwave monolithic integrated circuit (MMIC).
The artificial transmission lines are coupled together by the transconductance of the FETS. The drain line artificial transmission line 30 is connected to A/C ground by way of a drain line termination impedance 48, selected to match the characteristic impedance of the drain line 30. Similarly, the gate line artificial transmission line 28 is terminated at a gate line termination impedance 50, selected to match the characteristic impedance of the gate line 28. Many factors are known to influence the performance of such distributed amplifiers, such as the cutoff frequency of the selected FET, the cutoff frequency of the artificial transmission lines as well as the transconductance of the selected FET. Many attempts have been made to optimize these factors to provide an optimal gain bandwidth product characteristic of the device. For example, U.S. Pat. No. 5,559,472 discloses the use of a plurality of gain cells configured in a cascade configuration in order to improve the overall gain without decreasing the bandwidth performance of the device. Unfortunately, with such topology, each gain cell includes three or more bipolar transistors which increase the overall size, cost and complexity of the device.
It is known that increasing the passband of a distributed amplifier can result in instability (i.e. oscillation) of the amplifier at increased frequencies. As such, attempts have been made to improve the stability of the distributed amplifiers resulting from improved passbands. For example, U.S. Pat. No. 5,386,130 discloses a multi-stage distributed amplifier with improved gain. In order to improve the stability of the device a low-pass filter is connected to each stage to limit the passband to frequencies where the amplifier is stable. Unfortunately, reduction of the bandwidth of a distributed amplifier is highly undesirable.
Another consideration in such distributed amplifiers is the ability to provide a distributed amplifier topology with variable gain for use in various broad band signal processing applications. Unfortunately, the gain characteristic of the distributed amplifier topologies discussed above is fixed and determined by the transconductance of the FET device. However, variable gain distributed amplifier topologies have been developed which enable the gain of the device to be varied. Examples of such variable gain distributed amplifier technologies are disclosed in U.S. Pat. Nos. 4,918,401 and 4,947,136. The '401 and '136 Patents disclose distributed amplifiers formed from a plurality of segmented dual gate FETS (SDG FETS). Such SDG FETS have an additional gate terminal divided into selectible segments of predetermined widths. Since the transconductance and thus the gain of the device is proportional to the gate width, the gain of the device can be varied by selection of the segments. Although the devices as disclosed above are known to provide adjustable gain, such devices require relatively complicated processing techniques and are also relatively large compared to known distributed amplifiers.